Intonating a flattop’s bridge saddle is tricky, and that’s especially true when it comes to vintage 12-string guitars. Back in the day, it was rare to find an acoustic 12-string with a compensated bridge saddle. In fact for many players, the slightly out-of-tune jangle was a big part of a 12-string’s aural mystique. But times have changed, and as music and recording technology have evolved, most musicians and producers expect a 12-string to play reasonably in tune all along the fretboard.
Intonating a flattop 12-string with a straight bridge saddle involves filing unique break angle and intonation points for each string—an operation that requires skill and patience. Let’s investigate and see exactly what’s involved.
We’ll use a beautiful 1972 Martin D-12-20 as our project guitar. The guitar was in good shape when its owner brought it into the shop, but it didn’t play even remotely in tune and the action was uncomfortably high. Many old 12-strings require a neck reset (an expensive proposition), but I checked the neck angle and it was fine. Fortunately for the guitar’s current owner, the problem was rather simple. Somewhere along the line a previous owner or tech had installed a saddle that was both too high and incorrectly radiused (Photo 1).
My job was twofold: lower the action by reshaping the bridge saddle and then intonate each string. I knew that once I completed these tasks, the D-12-20 would play better than ever and sound more in tune at the higher frets.
Requisite tools. To intonate a 12-string, you need a few specialized luthier tools. These include a string action gauge, radius gauges, and a radius block (the latter must match the fretboard radius). You’ll also need a mechanical pencil, a capo, self-adhesive 80-grit and standard 400-grit sandpaper, and flat, single-cut miniature needle files.
Preliminary check. Before starting this kind of work on a guitar, it’s important to tune to concert pitch and take measurements. These specs provide a baseline to help gauge your progress. You’ll be taking several measurements, so write them down as you go.
First, put a capo on the 1st fret and measure action at the 12th fret with a string action gauge. Take this measurement for both the 1st and 11th strings—the first of the doubled high Es and the low E string. (We’ll assume your 12-string has the standard octave-string configuration, i.e., the octaves in each pair are closer to you than their wound partners. Some electric 12s, notably Rickenbackers, reverse this order.) Measure the distance between the bottom of the string and the top of the 12th fret.
Next measure the relief: With the capo still on the 1st fret, hold the 11th and 12th strings down at the 14th fret and measure the greatest gap between the bottom of the 11th string and the top of the frets. Typically this occurs around the middle of the fretboard, somewhere between the 7th and 9th frets. You can identify this gap by tapping the 11th and 12th strings against the frets while still pressing them down at the 14th fret.
Now remove the capo and check the action at the 1st fret.
Finally, using a strobe tuner, check the intonation for each string. (Naturally, this is tricky on a 12-string because you have to pluck each individual string of the six pairs.) Starting with the 1st string, play the 12th-fret harmonic—make sure it’s in tune—and then fret the same note. If the fretted note is sharp or flat compared to the reference harmonic, write down how many cents it’s off and in which direction. Repeat the process until you’ve documented the intonation at the 12th fret for all 12 strings.
Our 12-string’s preliminary specs. Here’s how the D-12-20 measured up: The action at the 12th fret was 6/64" for the 1st string and 7/64" for the 11th string. Too high to play! The relief was .012"—perfect for my client’s playing style. At the 1st fret, the 1st string was 1/64" and the 11th string was just over 2/64" above the fret. Again, perfect action at the string nut. At this point, I knew my job would simply entail lowering the action at the bridge saddle.
When I checked the intonation with a strobe tuner, and compared the 12th fret harmonics to their corresponding fretted notes, I found most of the fretted notes were from two to six cents sharp, although the G and D pairs were flat by about three cents.
With these measurements in hand, I was ready to get to work.
Sanding the bridge saddle. My next step was to check if the saddle’s radius matched the fretboard radius. My radius gauge revealed what I’d suspected—it wasn’t even close. The fretboard had a 14"radius (Photo 2), but the bridge saddle was around 8". This meant the saddle had a much more pronounced arch than the fretboard. I knew if I didn’t reshape the top of the saddle to match the fretboard, the D and G pairs would be radically higher than the other strings.
Using a 14" radius block and self-adhesive 80-grit sandpaper, I sanded the top of the saddle until it matched the block’s radius. Here’s the most accurate way to reshape the top of the saddle: Place the radius block in a vise with the radius side up and affix the self-adhesive sandpaper to the block. Now remove the saddle from its bridge slot, turn it upside down, and gently sand its top in the block’s concave area. If you shine light behind the saddle, you’ll be able to see how much material you’re removing from the saddle and make sure its radius matches the block (Photo 3).
Since the action was pretty high on our 12-string, I removed about 1/32" from the top of the saddle at both E string pairs. I removed quite a bit more from the middle of the saddle to flatten out the arch.
Once the saddle is correctly radiused, slip it back into the bridge and grab your mechanical pencil.